Acoustic sounding apparatuses, such as fish-finding echo sounders and scanning sonars, are conventionally used as means for detecting underwater fish schools (refer to patent document 1 “Japanese Examined Patent Publication No. 1973-26299” and patent document 2 “Japanese Unexamined Patent Publication No. 2003-202370,” for example). Installed on a ship, a fish-finding echo sounder transmits a beam of ultrasonic sounding waves (acoustic waves) vertically beneath the ship's hull to search for and detect fish schools existing below the ship. A scanning sonar, also installed on a ship, transmits a beam of acoustic waves into underwater areas surrounding the ship to scan the surrounding areas and detect fish schools present in these areas. A result of search carried out by such acoustic sounding apparatus is presented as a scanned image in the form of a specific cross section of a detected fish school.
From a fisherman's point of view, however, it is preferable to recognize not only a scanned image of a detected fish school in the form of a specific cross section but also fish quantity information about the fish school. To achieve this, the prior art proposes a scanning sonar provided with horizontal mode in which the scanning sonar scans a full-circle area as well as vertical mode in which the scanning sonar scans a generally vertical fan-shaped cross sectional area (refer to patent document 2, for example). This scanning sonar presents scanned images obtained in both the horizontal mode and the vertical mode. The fisherman recognizes the shape of a fish school in its entirety and estimates fish quantity information about the fish school. By comparison, a fish-finding echo sounder does not allow the fisherman to recognize the shape of a fish school in its entirety, because the fish-finding echo sounder transmits an acoustic sounding beam vertically downward from the ship.
It is however not so easy for a fisherman to recognize a relationship between the horizontal and vertical mode images with respect to the shape of an entire fish school, for example, based on the scanned images obtained in the horizontal mode and the vertical mode, and this skill of recognition greatly depends on experience of individual fishermen. Furthermore, if fish quantity information about a fish school is to be estimated based on the horizontal and vertical mode images, it is quite likely that different fishermen will differently judge the displayed images, resulting in variations in obtained data. Additionally, estimated values of the fish quantity information about the fish school thus obtained are no more than qualitative results.
The present invention has been made in light of the aforementioned problems of the prior art and, thus, it is an object of the invention to provide an underwater sounding apparatus which can calculate fish quantity information about a fish school in a quantitative manner with little variations as well as a method of performing such calculation regardless of experience of fishermen.